High pressure polymerization reactors are widely used for the manufacture of ethylene-based polymers, and include autoclave reactors which typically operate at between about 1220 to 2000 bar (122 to 200 MPa) and tubular reactors which typically operate at between about 2500 and 3100 bar (250 to 310 MPa). For both types of reactors, fresh monomer from a monomer supply is compressed to reactor pressure by the combination of a primary compressor which pressurizes the monomer to an intermediate pressure, and a second compressor which pressurizes the monomer together with monomer from the intermediate pressure up to a final reactor pressure. Both types of reactors create a product mixture comprising principally polymer and unreacted monomer. The mixture typically leaves the reactor through a high pressure let down valve, and then enters a separation system in which unreacted monomer is separated from the polymer and recycled back to the process.
A variety of ethylene-based polymers can be manufactured in high pressure processes. Ethylene-vinyl acetate copolymers (“EVA”) may be produced in high pressure reactors and are commonly used in films and other applications where clarity and gloss are important. The weight percent of units derived from vinyl acetate comonomer in EVA typically varies from as low as 1 wt % to as high as 40 wt %, with the remainder of the polymer being ethylene-derived units.
Polymerization of ethylene-based polymers is a highly exothermic process. The reaction initiation temperature, or the temperature at which polymerization is started may be from about 120° C. to about 240° C. The exothermic nature of the process can lead to maximum temperatures within the reactor of from about 160° C. to about 360° C. Consequently, there is a need to cool the product mixture after it has left the reactor to limit polymer decomposition and the formation of high molecular weight polymer, as well as to enhance monomer recovery and obtain a good pellet cut in downstream processing.
In the prior art, this cooling has been accomplished by injecting cold ethylene into the flow of product mixture from the reactor. This cools the product mixture as it enters the separation system, promoting phase separation of the product into a polymer-rich liquid phase and a monomer-rich gas. Before the cold ethylene can be injected, however, it must be compressed to a relatively high pressure that will enable such injection. To accomplish this, the required flow of cold, compressed ethylene has been diverted out of the flow of make-up ethylene that the primary compressor supplies to the second compressor. Typically, the discharge pressure of the primary compressor is set to a level that is at least equal to the suction pressure of the secondary compressor. This alone places a considerable demand on the primary compressor. When ethylene from the primary compressor is diverted to quench the product mixture, however, the discharge pressure of the primary compressor must be set to a level considerably higher than the suction pressure of the secondary compressor. Such a design substantially increases the energy and capacity requirements of the primary compressor, and thus adds significantly to the cost and complexity to the process.
This cooling has also been accomplished by a product cooler downstream of the high pressure let down valve and upstream of the product separation system. In the production of EVA in conventional high pressure processes, however, where a product cooler is located downstream of the high pressure let down valve, increased gel levels in the polymer and an increased tendency of the reactor system to foul have been observed. This is particularly so when the EVA product comprises moderate or higher levels of units derived from vinyl acetate comonomer, such as levels at or above about 6 wt %. Background references include US 2007/032614, WO 2012/117039, and WO 2015/166297.
Thus, there is a need for improved systems and processes for cooling ethylene-based polymer product mixtures in high pressure processes. Particularly, there is a need for improved systems and processes for cooling EVA product mixtures manufactured at high pressure, especially processes capable of minimizing gels in the product and mitigating fouling within the reactor system.